Optical transmission systems use Optical Supervisory Channel (OSC) for the purpose of controlling communication between optical nodes. For example, an optical node sends an OSC signal, combined with an optical communication signal carrying data channels, to a subsequent node located downstream. Typically, the receiving optical node amplifies the received optical communication signal up to a specified power level by using an optical amplifier at the front end (called pre-amplifier). The amplified optical communication signal is then subjected to an optical add-drop multiplexer (OADM) to drop some signals destined for a lower-level network and add some other signals from the lower-level network. The optical communication signal suffers a certain amount of attenuation during the course of this add and drop processing at the OADM. The optical node thus compensates for the signal attenuation by using another optical amplifier (called post-amplifier) to regain the specified power level before the signal is transmitted to the downstream optical node. The optical nodes having such OADM functions may be referred to as “OADM nodes” where appropriate in this description.
Some optical nodes in a network do not have post-amplifiers. For example, an in-line optical node (i.e., node without OADM capabilities) simply amplifies and forwards received optical communication signals to adjacent nodes, without adding or dropping any signals. The absence of OADM capabilities means that such nodes are free from the attenuation of optical communication signals due to add and drop processing and thus from the need for the attenuation compensation mentioned above. For this reason, in-line optical nodes use in-line amplifier units that include pre-amplifiers, but no post-amplifiers.
As an example of conventional techniques, there is proposed a communication method that provides high connectivity in an optical network (see, for example, Japanese Laid-open Patent Publication No. 2002-185482). The proposed technique offers a packet-switched optical ring network that combines the features of time division multiplexing and wavelength division multiplexing.
Under particular conditions, even OADM nodes are allowed to use in-line amplifier units and eliminate post-amplifiers. For example, this option may be applied in the case where the distance of the transmission line is so short that the optical transmission system can ensure the quality of optical communication signals without post-amplifiers. In this case, the OADM node using in-line amplifier units first amplifies a received optical communication signal with its pre-amplifier. The amplified signal then undergoes OADM processing and is transmitted as is (i.e., without post-amplification) to the next optical node.
The output power level of an OADM node using in-line amplifier units is lower than the output of an OADM node having a post-amplifier, because of the lack of additional amplification to recover the loss of power caused during add and drop processing in the OADM. In other words, the OADM node using in-line amplifier units produces an increased difference between the per-wavelength power level of an OADM-processed optical communication signal and the power level of an OSC signal transmitted together with the optical communication signal. To remove such an OSC signal from a received optical communication signal, the downstream optical node has to use, for example, a costly high-performance demultiplexing filter.